The present invention is directed, in general, to temperature detecting apparatus that can be applicable to electronic fire detectors and the like. In particular, this invention is related to an improvement in temperature detecting apparatus functioning to sense unusual increase rates of ambient temperature.
It is well known that a forward voltage drop developed across the P-N junction of a semi-conductor diode (called "forward voltage" hereinafter) will vary linearly over a wide range of temperature change when the forward current thereof (called "forward current" hereinafter) is kept constant. Hence, a semi-conductor diode is widely used as a temperature sensing element having stable and reliable characteristics. In FIG. 1, there is shown a sample of typical temperature-forward voltage characteristics of a semi-conductor diode.
Within the prior art, a variety of temperature detecting apparatus based on such characteristics of semi-conductor diodes are known. A review of those known devices makes it easier to understand the objects, advantages and features of this invention.
Referring now to FIG. 2, there is shown a schematic circuit diagram of a temperature detecting apparatus according to prior art, in which numeral 1 denotes a sensing circuit comprising a diode D.sub.1 and a resistor R.sub.1 that decides a forward current of the sensing circuit. Numeral 2 indicates a reference voltage generating circuit consisting of a resistor R.sub.2, variable resistors VR.sub.1 and VR.sub.2, and a Zener diode VZ.sub.1. The reference voltage generating circuit produces two kinds of predetermined voltage V.sub.1 and V.sub.2, where the voltage V.sub.1 has a closer value to the forward voltage V.sub.0 of the diode D.sub.1 of the sensing circuit 1. Numeral 3 denotes an amplifier circuit comprising an amplifier A.sub.1 and two resistors R.sub.3 and R.sub.4, which amplify the voltage difference between the output voltage V.sub.0 of the sensing circuit 1 and the output voltage V.sub.1 of the reference voltage generating circuit 2. Numeral 4 indicates a differentiating circuit comprising a capacitor C.sub.1 and a resistor R.sub.5, which produces an output voltage in proportion to the change rate of the output of the amplifier circuit 3.
Numeral 5 denotes a comparator circuit composed of an amplifier A.sub.2 functioning to compare the output from the differentiating circuit 4 with one of the outputs V.sub.2 of the reference voltage generating circuit 2, which will develop an output when the output of the differentiating circuit 4 becomes larger than the reference voltage V.sub.2. Numeral 6 indicates a switching circuit consisting of a thyristor SCR.sub.1 and two resistors R.sub.6 and R.sub.7, which are rendered conductive in response to the output from the comparator circuit 5 to energize a warning device 7. Graphical illustrations of operational states of each circuit as well as temperature variation with time are shown in FIG. 3.
A temperature detecting apparatus of the type described above taken in conjunction with the drawing shown in FIG. 2 usually has some problems directly connected with the inherent characteristics of a semi-conductor diode. First, the voltage change rate of the sensing diode caused by temperature variation is usually very small as compared with the forward voltage thereof. Hence, it is inevitably required to provide a precise voltage reference, not affected by temperature change, to detect errorlessly the difference between the reference voltage and the forward voltage of the sensing diode. A reference voltage less affected by temperature change can be obtained by employment of, for instance, a very expensive circuit element.
Second, since the forward voltage of a semi-conductor diode varies from one to another within certain limits, the variation must be compensated by means of adjusting the reference voltage in every apparatus. Hence, calibration of each device is necessary as a fundamental part of installation and use.
Another problem exists in the necessity of a change rate detecting circuit such as a differentiating circuit in order to detect temperature change rate, namely temperature rise rate. It is generally acknowledged that the differentiating circuit must be provided with at least several minutes of time constant in order to effectively detect fire. The realization of such a larger time constant necessitates larger values of the capacitor and resistor. Moreover, when such a larger time constant is employed for the differentiating circuit, it may take as long as the time constant of the differentiating circuit until the circuit reaches steady state operation after the power supply is turned on. This means that the apparatus will not be able to fulfill its primary function as a temperature detector, which is a crucial problem for an apparatus such as fire detector.
Also, in the prior art, temperature sensors for detecting fire or extreme heat conditions by a determination of absolute temperature are known. Typical is the circuit shown in U.S. Pat. No. 4,071,813. In that patent, the difference between base-emitter voltages of matched transistors is used to provide an output directly proportional to sensed absolute temperature. The two transistors are operated at different current densities and are maintained at the same temperature. Such a technique while generally effective still requires accurate matching of circuit elements and precise calibration. This is particularly true where the device is intended to exhibit a linear output directly related to a known temperature scale.
In contrast, the present invention does not require such precision in calibration and uses two transistors operating at the same current densities when the transistors are subjected to the same temperature. The advantages of the invention therefore lie in the use of the system for the detection of fire by sensing temperature differences.